An organelle is a unique part of a cell that has a specific function. The term is a combination of the word "organum," which means instrument or engine, and the suffix "-elle," which means small.
Both prokaryotic and eukaryotic cells are surrounded by a membrane of plasma that contains cytoplasm, which contains organelles and fluid. The organelles themselves are one of the most important cell components and aid in the overall function of the particular type of cells. Examples of organelles include the nucleus, mitochondria, and chloroplasts, which each have a specific function within the cell.
Not all cells contain every type of organelle since some are very specialized. Chloroplasts are found in organisms such as plants and algae and are responsible for the photosynthesis process. Some cells like those that comprise the pancreas contain storage granules, which are a type of organelle that store secretions until they receive the signal to release them. Microvilli are another type of organelle that increase the area of the surface of the cell and are for example found in the intestine; they form tiny protrusions and absorb nutrients.
Organelles are very small and difficult to detect. Even the larger type such as nuclei and mitochondria have not been studied in great detail because they their size is near the resolution limit of modern microscopes.